well the most easiest in few steps is to configure your ZD to ensure that AP's only know one ZD IP to connect to whatsoever...in ZD -- configure - AP - limited ZD discoveryjust fill primary ZD IP as your current ZD IP and for secondary as 0.0.0.0
It all depends. If SZ has LWAPP2SCG discovery function turned on, AP will auto join the SZ (Assuming it knows the IP of the SZ either via DHCP option 43 sub 3 or DNS, etc.) and the SZ will auto upgrade the AP to SZ code. However, if SZ code is 3.6.x, then this AP itself would not be supported. Any supported AP, running ZD code can be auto upgraded to SZ code by the SZ itself. All the AP needs is to know the IP and for the SZ to be running the LWAPP2SCG script. Since he wants to run both, old and new APs and have them go to their respective controller, his best bet is to set Option43 sub3 with IP of ZD and Option 43 sub 6 with IP of SZ. This way, ZD APs go to ZD and SZ APs go to SZ. This happens automatically and no need for AP or SZ or ZD specific config.